Two-way wireless messaging systems are known. As such messaging systems have evolved, there has been a need to communicate larger amounts of data to satisfy customer requirements. This need has brought about a requirement for higher speed modulation techniques, such as quadrature amplitude modulation (QAM). Higher transmission speeds result in a need for location of the portable subscriber unit for which a message is intended. This is because simulcast transmissions cannot be used at high transmission speeds due to the degrading effects of differential transmission delay on very short duration symbols. Instead, a non-simulcast transmission from a single transmitter close to the subscriber unit is required, thus forcing the need for location of the subscriber unit.
On the other hand, a small amount of simulcast transmission remains necessary, for example, to locate the subscriber unit when its position is unknown, or to send a system-wide command affecting all subscriber units. Simulcast transmissions have been utilized also for synchronization of the subscriber units throughout the messaging system. Unfortunately, simulcast synchronization signals alone do not have sufficient accuracy and resolution for synchronizing the very high speed transmissions possible with non-simulcast transmission.
Thus, what is needed is a method and apparatus for synchronizing the subscriber unit when higher speed modulation techniques are employed. Also needed is a method and apparatus for locating the subscriber unit.